1. Field of the Invention
The present invention relates to a cellular telephone equipment and a cell search method used therefor and, more particularly, to a cell search method for a CDMA (Code Division Multiple Access) cellular telephone equipment.
2. Description of the Prior Art
CDMA is one of conventional multiple access techniques used when a plurality of mobile stations simultaneously communicate in the same frequency band in a mobile radio communication system (cellular phone system) such as an automobile telephone system or portable telephone system. In CDMA, multiple access is achieved by spread spectrum communication which spreads the spectrum of an information signal to a band much wider than the original information bandwidth and transfers the information signal.
If a mobile station moves from cell to cell in a cellular phone system, CDMA executes, as hand-over, soft hand-over which synthesizes a signal from the base station of the cell the mobile station is leaving and a signal from the base station of the cell it is entering. To execute this hand-over, the mobile station performs cell search; it searches for the base station of the cell it is entering, i.e., cell search of detecting a connection cell.
In CDMA, upon reception of an incoming signal to the mobile station, the base station must notify the mobile station of a message to this effect. The base station broadcasts the incoming signal to all mobile stations in an area in which the mobile station is registered. This procedure is called paging.
In a standby state, the mobile station always monitors a ringing channel (paging channel), and can recognize paging to the mobile station itself. The mobile station compares paging information with information held by it, and if they coincide with each other, sends a response to a base station.
In cell search operation of the CDMA cellular telephone equipment, the cellular telephone equipment in a standby state classifies cells into two types: “cells detected in previous cell search” and “peripheral candidate cells”. These cells are searched at once at the paging channel reception timing.
The above-described cell search operation of the conventional cellular telephone equipment searches at once the two types: “cells detected in previous cell search” and “peripheral candidate cells” every paging channel reception timing. The cellular telephone equipment exhibits high search ability, but suffers a long search time and large current consumption.
Even if the search ability is lowered to shorten the search time, the search ability and search time cannot be flexibly adjusted because there is proposed only one search method of classifying cells into two types: “cells detected in previous cell search” and “peripheral candidate cells”.
“Cells detected in previous cell search” may be divisionally searched, but the search frequency of a cell which is receiving a paging channel decreases.